In counter-currently contacting, gas flows upwards and liquid flows downwards through the column. To allow contacting of upwardly flowing gas and downwardly flowing liquid, the column is provided with a plurality of horizontal contact trays arranged axially spaced apart in the column, wherein each contact tray is provided with gas passages and a downcomer which opens below the contact tray.
In the specification and in the claims the word `downcomer` is used to refer to a conduit for downward transport of liquid from a tray, wherein the inlet end of the conduit is provided with a weir extending above the tray. The weir can be part of the downcomer or the weir can be separately arranged on the tray.
During normal operation, gas flows upwards through the gas passages of the contact tray and liquid is supplied onto the contact tray through the downcomer of the next higher contact tray. Liquid collected on the tray forms a layer and contacting of gas and liquid takes place in the layer of liquid on the contact tray. Liquid flowing over the weir is removed through the downcomer, and gas disengaged from the liquid on the contact tray flows upwards to the next higher contact tray on which it is again contacted with liquid.
To reduce the amount of liquid entrained with the gas that flows to the next higher contact tray it has been proposed in U.S. Pat. No. 1,983,762 to Kotzebue to provide the column with a plurality of horizontal separation trays provided with swirl tubes and with means for removing liquid from the separation tray, wherein each separation tray is arranged above a contact tray. The next higher contact tray is then arranged above the separation tray.
The swirl tube is a cylindrical pipe in which swirl imparting means are arranged. During normal operation, gas with entrained liquid flows through the swirl tube and the swirl imparting means cause the gas to rotate; under influence of centrifugal forces, entrained liquid moves away from the center of the swirl tube and is collected on the inner surface of the cylindrical pipe where a film of liquid is formed. At the upper end of the pipe, the liquid film breaks up and liquid droplets move away from the pipe; these liquid droplets should fall on the separation tray. The path of some of the liquid droplets will be such that they hit the contact tray above the separation tray, and they are entrained through the gas passages of that contact tray. To reduce this entrainment, the upper ends of the swirl tubes discharge straight into the gas passages of the contact tray above the separation tray, so that the gas flows towards the gas passages of the contact tray and the liquid hits the contact tray around the gas passages. Also, the known separation tray comprises as many swirl tubes as there are gas passages in the next higher contact tray, and the diameter of the swirl tubes is equal to the diameter of the gas passages in the next higher contact tray. And the layout of the swirl tubes is similar to the lay out of the gas passages, consequently the net free area of the known separation tray equals the net free area of the contact tray, wherein the net free area of the separation tray is the area of the passages in the swirl tubes divided by the active area of the separation tray, and wherein the net free area of the contact tray is the area of the gas passages divided by the active area of the contact tray.
It is an object of the present invention to provide a separation tray which contains a much smaller number of swirl tubes which have a diameter that is larger than the diameter of the gas passages.